1. Field of the Invention.
The present invention relates to helicopters generally and in particular to vibration reduction therein.
2. Description of the Related Art.
The reduction of vibrations is a primary goal in helicopter design. Such vibrations contribute to crew fatigue, passenger discomfort, increased maintenance, and high operating costs. A major cause of such vibrations is periodic aerodynamic loads on the rotor blades.
An effective method of reducing rotor-blade induced vibrations is to control the harmonic airload at the source, i.e. on the rotor blades. For an N-bladed rotor, harmonic loads at (N-1) per revolution, N per revolution and (N+1) per revolution are transmitted to the rotor hub at the frequency rate of N/revolution (hereinafter NP). The function of HHC devices is to generate additional airloads on the rotor so as to cancel the three higher harmonics of blade load causing the NP vibratory hub load. Experience has shown that the NP blade flapping moment is the dominant load as compared with the (N.+-.1)P components.
Various schemes for reducing helicopter vibrations by HHC of airloads on the rotor blade have been investigated and disclosed by others. Some approaches are based on passive vibration control concepts involving dynamically tuned mechanisms which actuate either the swash plate or the tab surfaces on the rotor blade. Other HHC concepts make use of high frequency active control systems which, when coupled with vibration sensors, provide vibration reduction by either manual control or closed loop feedback control.
The most common approach for implementing an HHC system uses broad-frequency band actuators to drive feathering of the rotor blades at high frequencies. The actuators produce the desired blade motions by driving either the swash plate itself or the pitch links between the blade and the swash plate.
Another HHC system involves individual blade control and is commonly known as the IBC system. By the IBC system, the pitch of each rotor blade is controlled independently by a servo actuator responding to feedback control signals from sensors mounted on each rotor blade.
Other HHC systems use either blade servo-flaps or blade circulation control devices to dampen the vibrations.
Typical HHC systems are disclosed by the following:
Arcidiacono in his U.S. Pat. No. 3,649,132 which was issued on Mar. 14, 1972; by Campbell in his U.S. Pat. No. 4,514,143 which was issued on Apr. 30, 1985; by Fradenburgh in his U.S. Pat. No. 4,655,685 which was issued on Apr. 7, 1987; by Fischer, Jr. et al. in their U.S. Pat. No. 4,953,098, which was issued on Aug. 28, 1990; and by Fischer Jr. in his U.S. Pat. No. 4,965,879, which was issued on Oct. 23, 1990.
All of these known prior art devices have one or more of the following disadvantages: first, large amounts of force and power are required for high frequency actuation of blade pitch; second, the service life of blade pitch links is decreased due to increased fatigue; third, debilitating heat is generated by broad band electro-hydraulic actuators; fourth, blade circulation control devices are very complex and expensive to manufacture; and fifth, difficulties arise in tuning passive devices to be effective over a broad range of operating conditions.
Thus, it remains a problem in prior HHC systems to develop a system which requires small amounts of actuator torque and power, has a long service life, does not generate debilitating heat, is simple in design, is relatively inexpensive to manufacture, and avoids the difficulties arising from tuning passive devices.